A Tenth Planet?

In recent years astronomers have discovered that our solar system is a great deal more crowded than we imagined. In 2004 alone, a dozen new moons circling Saturn were identified. Today's sophisticated telescopes have also uncovered a host of large objects inhabiting the frozen edges of our solar system—Varuna (2000), Ixion (2001), Quaoar (2002), and Sedna (2004) are among the most spectacular of what are called Kuiper Belt Objects (KBO), masses of rock and ice that appear beyond Neptune.

But just when we've grown accustomed, perhaps even a little jaded, by the steady parade of new moons and KBOs discovered each year, astronomers have succeeded in truly rattling our notion of the solar system: in July 2005 the California Institute of Technology and NASA announced the existence of a tenth planet. About 9 billion miles from the Sun resides 2003 UB313, estimated to be about one-and-a-half times the size of Pluto. “We are 100% confident that this is the first object bigger than Pluto ever found in the outer solar system,” said Caltech astronomer Michael A. Brown, one of the three scientists responsible for the discovery.

The assertion of a tenth planet, however, is not without controversy, reviving the debate about what defines a planet and again calling into question whether the diminutive Pluto fits the bill. Some scientists consider the newly discovered object—along with Pluto—nothing more than a glorified Kuiper Belt Object. Alan Boss, a planet-formation theorist at the Carnegie Institution of Washington, contends that “to just call them planets does an injustice to the big guys in the solar system.” But Brown maintains, “Pluto has been a planet for so long that the world is comfortable with that. It seems to me a logical extension that anything bigger than Pluto and farther out is a planet.” When this long-standing controversy is finally resolved, it seems we will have either ten planets in our solar system, or just eight.

Given that 2003 UB313 is significantly larger than Pluto, why did it take so long to discover? According to Brown, its orbit lies 45 degrees from the plane on which the nine planets orbit the Sun, and until now “nobody look[ed] that high up in the sky.” The object is so bright that even amateur astronomers can spot it fairly easily.

The object was first identified on Jan. 8, 2005, and Brown and his team had planned on announcing the discovery once they had accumulated more definitive data on the object's size, mass, and composition. But when someone hacked the team's website, threatening to preempt them, Brown and the others rushed to get the news out on July 29. Brown has already submitted a name for the planet to the International Astronomical Union, but he plans to keep it a secret until it is officially accepted.